Processing of papermaking fibers to remove contaminants is well known in the art, including the use of forward cleaners and flotation cells. Such technology is used, for example, to treat secondary (recycle) fiber sources for re-use in paper products such as towel and tissue, paperboard, coated writing and printing papers and so forth. Equipment utilized includes screening devices, flotation cells and the like as may be seen, for example, in U.S. Pat. No. 4,272,315 to Espenmiller; U.S. Pat. No. 4,983,258 to Maxham; U.S. Pat. No. 5,240,621 to Elonen et al.; and U.S. Pat. No. 5,693,222 to Galvan et al.
Recycling paper into secondary pulp suitable for re-use in high quality products is a relatively complex, capital intensive undertaking as will be appreciated from U.S. Pat. No. 5,587,048 to Streisel et al. The basic cleaning sequence of the '048 patent is as follows: (1) detrashing—the detrasher contains 6 mm (¼ inch) holes and retains large contaminants, such as plastic bags, pieces of wood, large staples, pieces of metal and packing tape, detrashing typically takes place at 3-5% solids; (2) high-density cleaning—heavy, coarse contaminants, such as bolts, staples and rocks are removed, high density cleaning typically takes place at about 3-4% solids; (3) primary coarse screening—primary coarse screens contain holes 2-3 mm in size, preferably 2.4 mm, for removing medium-sized contaminants, such as small fragments of wood, tape and styrofoam, coarse screening at this stage protects fine slotted screens downstream from being overwhelmed by contaminants that are large relative to the slot width, and results in improvement in quality and production rates, coarse screening typically takes place at about 2.5-3.5% solids; (4) secondary coarse screening—the rejects from the primary coarse screening may be screened again using holes of the same size, but at a lower consistency, about 1.5-2.5% solids; (5) sand cleaning (centrifugal)—sand cleaning at this stage protects the fine slotted screens downstream from excess wear, waste corrugated paperboard contains relatively large amounts of sand, cleaning ahead of the screen increases the cost of the system, and increases the requirements for hydraulic capacity, sand cleaning typically takes place at about 1% solids; (6) screening—fine slotted screens are used with a width of 0.008 inch (0.20 mm), rather than 0.012 inch previously used for corrugated paperboard, the fine screens remove plastic slivers, wax and stickie agglomerates, screening takes place at less than 1% solids, preferably less than 0.9%; (7) Lightweight Cleaning (Gyrocleaning)—lightweight cleaning preferentially removes materials with a specific gravity below 1.0, such as plastics, waxes and stickies, not heretofore removed, lightweight cleaning is performed at about 0.8% solids.
It should be appreciated from the '048 patent that existing methods for handling stickies removal were based on removing light contaminants having a density generally less than the fiber being cleaned. Such methods have been found inadequate when a significant amount of heavy stickies are present.
The disclosures of the foregoing patents are hereby incorporated by reference.